Countries which export more than they import (i.e., which have a surplus in their balance of trade) still will have a $0 balance of payments. what is one way this occurs?

They trade so muh stuff and end up with nothing to profit from

The correct answer is "to function properly"

This is an infinitive phrase that consists of the infinitive "to function", and the adverb that serves as a modifier that is "properly". To be charged could also be treated as an infinitival phrase but here it is in its passive form and doesn't have the same function as the "to function properly" phrase.